1. Field of the Invention
The present invention relates to an internal combustion engine having a variable valve timing mechanism (hereinafter simply referred to as VVT) that variably controls the open/close timing (hereinafter simply referred to as valve timing) of an intake valve and an exhaust valve of the internal combustion engine in response to the operating state of an engine, and more particularly to a valve timing control device for controlling the operation of the VVT in its abnormal state.
2. Description of the Related Art
A conventional valve timing control device for use in an internal combustion engine includes, as shown in FIG. 9, an oil pump 1 for forcedly feeding a lubricating oil to a valve timing control system, a cam shaft 2 that is coupled to a crank shaft (not shown) of an engine by means of a power transmission mechanism such as a timing belt and is driven to rotate in synchronism with the rotation of the crank shaft to cause the intake valve and the exhaust valve to open and close through a valve operating mechanism, and a variable valve timing control hydraulic actuator 3 (hereinafter simply referred to as "VVT actuator") for varying the rotational phase of the cam shaft with respect to the crankshaft. The conventional valve timing control device also includes an oil control valve 4 for adjusting the amount of lubricating oil in the form of an operating oil supplied to the VVT actuator 3, a crank angle detector 5 for detecting the rotational phase of the crank shaft, a cam angle detector 6 for detecting the rotational phase of the cam shaft, and an engine operating state detector 7 for detecting the operating state of the internal combustion engine. The conventional valve timing control device further includes an oil control valve control circuit 8 that electrically drives the oil control valve 4, an electronic control unit (ECU) 10 that gives an instruction or command to the oil control valve control circuit 8, an oil temperature detector 14 for detecting the temperature of the lubricating oil supplied to the oil control valve 4, and a hydraulic pressure detector 15 for detecting the hydraulic pressure of the lubricating oil.
Subsequently,. the operation of the above-described conventional valve timing control device will be described. The ECU 10 calculates the current open/close timing (hereinafter simply referred to as "valve timing") of the intake and exhaust valves on the basis of an output of the engine operating state detector 7 that detects the operating state of the internal combustion engine. The ECU 10 calculates the operating amount of the oil control valve 4 in accordance with the hydraulic pressure of the lubricating oil so as to reduce a deviation between an optimum valve timing and the current valve timing, and outputs an instruction or command to the oil control valve control circuit 8. The oil control valve control circuit 8 adjusts the supply voltage or current to the oil control valve 4 so that the operating amount instructed by the ECU 10 and the electric behavior of the oil control valve 4 coincide with each other.
The lubricating oil forcedly fed from the oil pump 1 is controlled by the oil control valve 4 and then supplied to any one of a spark retarding chamber 3a and a valve timing advancing chamber 3b of the VVT actuator 3 by an appropriate amount in a desired direction. On the other hand, the lubricating oil, filled in the other chamber to which the lubricating oil is now not being supplied, is returned to an oil pan 12 through a drain 11 of the oil control valve 4.
When the oil is supplied to the spark retarding chamber 3a or to the valve timing advancing chamber 3b of the VVT actuator 3, a rotor 3c within the VVT actuator 3 is driven to rotate in a direction toward a valve timing advancing side or a spark retarding side due to hydraulic pressure. The cam shaft 2 is coaxially connected to the rotor 3c, and the rotation of the rotor 3c allows the rotational phase of the cam shaft 2 to vary with respect to the rotational phase of the crank shaft to thereby change the valve timing.
Subsequently, a control method or operation of the control device or ECU 10 that estimates the drive force of the VVT actuator 3 on the basis of the operating state of the engine and controls the valve timing in response to the drive force will be described with reference to a flowchart shown in FIG. 10.
First, the ECU 10 reads various sensor signals from the operating state detector 7, the crank angle detector 5 that detects the rotational phase of the crank angle and the cam angle detector 6 that detects the rotational phase of the cam shaft 2 (step S1), and calculates the current valve timing .theta. from the rotational phase of the crank angle and the rotational phase of the cam shaft 2 thus read (step S2).
Also, in step S3, an optimal valve timing .theta.T (hereinafter referred to as "target valve timing") in the engine operating state is calculated from the sensor signals (for example, the number of revolutions per minute (rpm) or rotational speed of the engine, the throttle opening degree, the charging efficiency, the temperature of an engine coolant or cooling water, etc.) indicative of the engine operating state as read in step S1.
In addition, the ECU 10 reads an oil temperature from the oil temperature detector for detecting the temperature of the lubricating oil (hereinafter referred to as "oil temperature") and estimates the viscosity of the lubricating oil from the oil temperature thus read in (step S4).
Subsequently, in step S5, the ECU 10 calculates a deviation .theta.e of the current valve timing with respect to the target valve timing and estimates a force (hereinafter referred to as "an operating force Fc of the cam shaft 2") necessary for operating the cam shaft 2 in order to set the deviation .theta.e to zero, from the viscosity of the lubricating oil as estimated in step S4 and the rotational speed (rpm) of the engine.
In step S6, the hydraulic pressure is read from the hydraulic pressure detector 15 that detects the supply pressure of the lubricating oil (hereinafter referred to as "hydraulic pressure") and then a cam shaft drive force Fp for driving the cam shaft 2 to rotate is determined from the hydraulic pressure supplied to the VVT actuator 3, to thereby estimate a force (hereinafter referred to as "ACT drive force Fa") by which the VVT actuator 3 changes the rotational phase of the cam shaft 2 with respect to the rotational phase of the crank shaft, where Fa=Fp-Fc.
In step S7, the operation of opening (when .vertline..theta.e.vertline.&gt;.theta.D) or closing (when .vertline..theta.e.vertline..gtoreq..theta.D) of the oil control valve 4 is determined on the basis of the relation in magnitude between the deviation .theta.e and a dead zone .theta.D. The dead zone .theta.D may b set to "0".
Then, In step S8, the operating amount for opening the oil control valve 4 is determined. The operating amount is determined on the basis of the deviation .theta.e calculated in step S5 and the ACT drive force Fa estimated in step S6. For example, if PID control is applied in the determination of the operating amount, respective gains are set in accordance with the ACT drive force Fa.
In step S9, the operating amount for closing the oil control valve 4 is determined through the same operation as in step S8.
In step S10, the operating amount determined in step S8 or S9 is converted into an electric signal by means of the oil control valve control circuit 8 to thereby drives the oil control valve 4.
In the control device that estimates the ACT drive force Fa and controls the valve timing in response to the ACT drive force Fa as described above, the operating amount of the oil control valve 4 is determined by multiplying the deviation between the target valve timing advancing amount and the actual valve timing advancing amount by a control gain which is set in response to the estimated ACT drive force. In general, a response time of the VVT actuator 3 tends to increase as the ACT drive force decreases, so the control gain is set to be large with the decreasing ACT drive force in order to provide stabilized controllability regardless of fluctuations of the ACT drive force (see FIG. 3).
However, because the hydraulic pressure depends on the temperature of the lubricating oil and the rotational number or speed of the engine, there is a case in which the hydraulic pressure temporarily remarkably becomes low depending on the operating state of the engine, and the ACT drive force becomes in the vicinity of or not greater than zero (0). In other words, this is a case of Fp&lt;Fc. Also, even if a sufficient ACT drive force exists immediately before the VVT operation, there is a case where the ACT drive force cannot sufficiently be ensured immediately after the VVT operation due to a reduction of the hydraulic pressure accompanied by the VVT operation. If the VVT is operated in this state, there arises such a problem in that the response of the actual valve timing advancing amount with respect to a change in the target valve timing advancing amount is remarkably lowered, or that the valve timing advancing amount cannot be held to a desired valve timing advancing amount.
Also, in the case where the valve timing cannot be set to a desired valve timing advancing amount, combustion in the internal combustion engine may become unstable or drivability and exhaust gas emission may be deteriorated.
Further, there may be a case in which abnormal abrasion occurs in the VVT actuator because the VVT actuator heavily vibrates in the vicinity of the most spark retarding position.
In addition, the VVT system having the hydraulic pressure detector 15 suffers from such a problem that it cannot detect an abnormality (for example, leakage of the lubricating oil from the VVT actuator due to wearing or the like) at the downstream of the hydraulic oil detector 15 although it can detect a hydraulic pressure abnormality at the upstream of the hydraulic pressure detector 15.